Abstract

Background

Satellite tagging programs have provided detailed information about the migratory
patterns of northeastern Pacific white sharks, revealing a seasonal migration between
a vast offshore region and coastal aggregation sites. Although adult males undergo
annual round-trip migrations, photo-identification programs have noted that sexually
mature females may only visit coastal aggregation sites once every 2 years, a behavior
that is presumably linked to an estimated 18-month gestation period. The whereabouts
of females during their full 2-year migration were previously unknown, because of
the limited battery capacity of satellite pop-up tags.

Results

Through the use of satellite-linked radio-telemetry tags with multi-year tracking
capability, we describe the 2-year migratory pattern for four mature female white
sharks tagged at Guadalupe Island, Mexico. The 2-year migration comprised four phases:
1) an Offshore Gestation Phase (which had an average duration of 15.5 months; 2) a
Pupping Phase, which occurred along the Mexican coast between the months of April
and August; 3) a Pre-Aggregation Phase (when the females were in transition between
the Pupping Phase and Guadalupe Island; and 4) the Guadalupe Island Aggregation Phase,
which began when the mature females arrived at Guadalupe Island between late September
and early October.

Conclusions

Long-term satellite tracking of mature female white sharks highlighted the connectivity
between a single presumed mating site at Guadalupe Island, and two widely separated
pupping sites along the Mexican coast. The Offshore Gestation Phase provided evidence
that the females remained offshore for up to 16 months during their 2-year migration
cycle. The Pupping Phase along the Mexican coast coincided with the seasonal presence
of young-of-the-year white sharks along the coast of North America, and with a presumed
gestation period of 18 months, this placed mating between October and January, during
the period when white sharks are known to be at Guadalupe Island. Tracking data during
the time sharks were offshore showed that mature males and females are spatially segregated,
except for their concurrent seasonal presence at Guadalupe Island. These discoveries
provide important new details about the complete life history of northeastern Pacific
white sharks while identifying crucial regions in which young-of-the-year, juveniles
and adult females are most vulnerable.